Compressed air is used in electrical switch gear primarily for quenching arcs in gas blast switches and for mechanical actuation of switchgear such as power circuit breakers, disconnecting switches and the like. Such switches require a supply of clean and dry air in order to insure a proper functioning of the switchgear.
A method to produce compressed air is known in which the humidity contained in the atmospheric air is extracted by means of compression and subsequent cooling. A condensation product which accumulates during this process and which emulsifies in part with the lubricating oil is discharged either at the time of shutdown or after a certain period of operation by means of unloader valves which are arranged at the various compressor states. In addition, it is necessary to provide a drainage system for the storage tank which stores the compressed air and is arranged successively to the compressor. The condensation product accumulating in the storage tank must be removed from time to time either manually or by means of a costly automatic system. Such a compressed-air generator which includes both a water separator and drainage systems requires the use of technical equipment that is costly and must be serviced quite frequently due to the formation of sludge caused by resinification of the water/oil emulsion, corrosion of the valves and seals, and the need to remove carbonized components of the lubricating oil from the water separator. Experience has shown that approximately 50% of the breakdowns of compressed-air generators are caused by the drainage system. Finally, a heating device is needed to prevent the freezing of the condensation product.
In another known method for producing and drying compressed air, the compressed air is supercooled at the high-pressure side so that the air will be very dry at the user side. The condensation product accumulating within this system is removed by special water separators which are equipped with electric or pneumatic unloader valves. Operating costs of this method are high when a high compression pressure is obtained.
An object of the present invention is to provide a greatly simplified method to produce and dry compressed air.
Another object of the invention is to provide a drying apparatus for the practical application of the method of the present invention.
In the present invention air is being drawn in and is pre-dried by a first stage of a first drying process prior to its entry into the compressor, with the temperature of the air being kept above the dew point during compression and cooling. The air is then heated prior to a second drying process with the remaining moisture being removed during the course of this second drying process so that the entire humidity of the air is extracted in the steam phase.
The first drying process may be improved still further by subdividing the process into several drying stages. A first drying stage is carried out prior to the compression process with several drying stages being intermixed with the compression process and the final compression stage followed, if necessary, by a last drying stage. These various steps are accomplished in this manner if the compressor has several compression stages.
Any oil that is possibly present in the air in the form of oil droplets, is gasified in the course of the process at the time when the air is being heated prior to the second drying process and is then extracted from the air in the gaseous state.
A drying apparatus, according to the present invention, may comprise a cylindrical inner housing and an outer housing, which surrounds the former in jacket form. The outer housing is used for the pre-drying of the air, and the inner housing is used for the final drying. The inner housing has a cover plate at each of its two ends. One of the cover plates is followed by an additional housing which acts as a desorption volume and is connected with the inner housing by a filter and a one-way valve. A cylindrical hollow body, which accomodates an electric heating rod, protrudes concentrically through the other cover plate into the inner housing. A pipe, that is closed off on one side, is placed over the hollow body so that a ring-gap is created between two components, with the ring-gap divided into two halves. In this way, the compressed air is guided along the heating rod for intensive heating prior to the final drying.
An advantage of the invention is that the process, while still being very efficient, requires a substantially smaller technical outlay than the known methods because the moisture of the air is removed from the compressed air in the vapor state. Another advantage is that there is therefore no need for water separators and unloader devices nor for tanks connected to drainage systems for the removal of condensation products as required by all known methods. For this reason it becomes possible to substantially reduce the dimensions of the compressed air system. The extensive removal of the moisture from the air prior to its entry into the first compression stage of the compressor avoids a saponification, or respectively, a premature aging of the oil. The individual compressor stages can operate at reduced running time with a significantly lower compression. The compressor is no longer subject to corrosion and servicing is reduced to a minimum. In cases where compressed air of a specific relative humidity is required, as is possible for example if the switchgear consists of components made of plastics, humid air may be added to the compressed air in the amount desired. It has been found that this method is less complicated than the removal of humidity from the compressed air until a specific degree of relative humidity has been reached.